A semiconductor chip, also commonly referred to as an integrated circuit (IC) chip or a die is typically assembled into an IC chip package that is soldered to a printed circuit board. The IC chip package typically includes the IC chip, which contains a number of round solder bumps that are attached to a top surface of the chip. The IC chip, via the solder bumps, is soldered to solder pads located along a surface of a package substrate, forming a metallurgical joint between the chip and the substrate referred to as a C4. C4s carry electrical current between the semiconductor chip and the substrate.
Power density in new technologies has increased dramatically, which in turn increases the current flowing through each C4 in a flip chip package. The amount of current flowing through each C4 has placed limitations on chip fabrication as a result of C4 electromigration. C4 electromigration refers to the transport of material as a result of the gradual movement of ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. As atoms are transported, voids are created in the C4, leaving the C4 unable to perform its function as a conductor.
Electromigration occurs as a function of current density, making it a particular challenge in applications with particularly high current and/or particularly small C4s, each of which increases the current density. Particularly as technologies progress to the 14 nm technology node and beyond, electromigration in C4s becomes more and more of a bottleneck.
Silver content in the solder of a C4 has been shown to impact the electromigration performance of the C4. Increasing the silver content improves the electromigration performance, i.e., it decreases the occurrence of electromigration. However, increasing silver content also results in a stiffer C4, placing more stress on the chip and therefore increasing the risk of “white bumps,” where the C4 exerts greater stress on the chip than the back end of line (BEOL) can withstand, resulting in a tear-out of the BEOL. Higher Ag content also limits the reflow profile options during assembly.